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Expedition 311 Scientists2
Site U1326 (proposed Site CAS-03C; Collett et al., 2005) is located on top of the first uplifted ridge of accreted sediments along the margin-perpendicular transect established during Integrated Ocean Drilling Program (IODP) Expedition 311 (Fig. F3 in the "Expedition 311 summary" chapter). The newly acquired bathymetry data from the University of Washington show a collapse structure near the originally proposed Site CAS-03B (Collett, Riedel, Malone, et al., 2005) that was previously unrecognized (Fig. F5 in the "Expedition 311 summary" chapter). A map with the site survey seismic data acquired in the area is shown in Figure F1. We switched former alternate Site CAS-03C to the primary site to avoid coring into the slump feature because it may locally complicate the history of deposition, fluid flux, and related gas hydrate formation.
The head wall of the slump feature is ~250 m high and the slump has eroded a ~2.5 km long section into the ridge. Slump material can be identified in the bathymetry data and previously in SeaMARCII acoustic imagery (Davis et al., 1987). Because of the steep slope, it is difficult to seismically image the deposit. A striking characteristic of the ridge is the occurrence of several linear features crossing the ridge in an east–west direction (Fig. F5 in the "Expedition 311 summary" chapter). These linear features are clearly associated with faults as seen in Figure F2. The faults outcrop at the seafloor and generate a seafloor displacement of as much as 25 m. These faults can be seismically traced from the surface down through the sedimentary section to depths below the bottom-simulating reflector (BSR). The occurrence of the faults is limited to the location of the slump scar, and the ridge heals to either end at the southeast and northwest limits, where the sediments are not faulted but appear seafloor parallel. Overall, the seismic character of the ridge changes from the southwest to the northeast across the ridge, with the southwest-facing part of the ridge characterized by strong, semicontinuous reflectivity (Figs. F3, F4), whereas the seismic reflectivity disappears underneath the northeast-facing flank of the ridge (Fig. F5). The slopes on the flanks of the ridge are relatively steep, making seismic imaging challenging, but the ridge is generally symmetrical so the differences in the seismic character of the ridge cannot be explained by simple variable acquisition parameters. It is possible that the internal deformation of the sediments may increase toward the northwest-facing flank and results in a loss of seismic coherency similar to that observed in accreted sediments along this margin.
A BSR is present underneath most of the ridge, especially seen in the lower frequency seismic data (Fig. F3). However, the multichannel seismic data along Line PGC9902_CAS03a show that the BSR is practically absent underneath the slump feature where the most heavily faulted sediments occur. As a result of the complicated nature of the seafloor, imaging capabilities may be limited especially in the high-frequency data (see Fig. F4 in the "Site U1328" chapter).
The coring and logging objectives at this site were tied to completing the transect of scientific drill sites across the northern Cascadia margin near Vancouver Island. Site U1326 is the closet location to the deformation front and probably represents the tectonically youngest occurrence of gas hydrate on the northern Cascadia margin.
At this western end-member site of gas hydrate evolution in the accretionary prism, the objectives include
The operational plan to achieve these objectives was based on a general three-hole concept, which includes